According to the Victorians the bay leaf represents strength, but it's probably not that inspiring as a tattoo idea on its own. The Chinese see bamboo as a symbol of strength, as it bends but doesn't break. The gladiolus is said to mean strength of character, as well as love at first sight, and it's visually stunning too.
BORAGE
the flower-Iris
Heliotrope represents faithfulness which is a synonym of loyalty.
a dead looking camomile
The Hydrangea is a Chinese flower and represents expressing love, gratitude and enlightenment. It is a good choice of flower to give as a gift.
BORAGE
Gladiolus represents emotional stability. It also represents strength of character and sincerity. There are around 260-species of gladiolus. The large, showy flowers are often used in flower arrangements.
Bermuda has a variety of symbols. Their bird is the Cahow because it represents strength, and endurance. Their flower is the Bermudiana, and their vegetable is the onion.
Is the flower you choose ;)
the flower-Iris
The bear or bear claw represents strength and resiliance.
I have not heard of a flower symbolizing strength, except perhaps for the Chrysanthemum which was a symbol of Imperial Japan.
Heliotrope represents faithfulness which is a synonym of loyalty.
There are many symbols that could be used to represent strength. A lion for example is an animal that represents strength.
Sunflower
Coral rose
The globe thistle.